Primary Menu

COMPRESSIBILITY FACTOR

COMPRESSIBILITY FACTOR

The compressibility factor (Z) or compressibility accounts for non-ideality in pressure, volume and temperature relation of an ideal gas given by the ideal gas law. The ideal gas law is given below:

P.V = n.R.T

where

P is pressure of ideal gas,
V is volume of ideal gas,
n is number of moles of ideal gas,
R is universal gas constant and
T is temperature of ideal gas

Compressibility factor is the ratio of real molar volume of a gas to ideal molar volume of the same gas at same temperature and pressure.

Z = Vm / (Vm)ideal gas

So ideal gas law is modified for real gases by using compressibility factor as given below:

P.V = n.Z.R.T

where

P is pressure of real gas,
V is volume of real gas,
n is number of moles of real gas,
Z is compressibility factor of real gas
R is universal gas constant and
T is temperature of real gas

For a gas that is a mixture of two or more pure gases (air or natural gas, for example), the gas composition must be known before compressibility can be calculated. Compressibility factor values are usually obtained by calculation from equation of state (EOS), such as the Peng-Robinson cubic equation of state, Soave Redlich Kwong cubic equation of state or their variants. Equation of state (EOS), such as the Peng-Robinson cubic equation of state, Soave Redlich Kwong cubic equation of state or their variants can give compressibility factor values for real gases as well as real liquids. Vapour compressibility factor values or gas compressibility factor values can be used to modify ideal gas law for real gases whereas liquid compressibility factor values can be used similarly in place of vapour compressibility factor values to give a useful pressure, volume and temperature relation for real liquids.